1. Field of the Invention
The present invention relates to an optical tomography system that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement.
2. Description of the Related Art
Conventionally, optical tomographs that utilize OCT measurement are employed to obtain tomographic images of living tissue. In an optical tomograph, a low coherence light beam emitted from a light source is divided into a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected by a measurement target when the measuring light beam is irradiated onto the measurement target, is combined with the reference light beam. Tomographic images are obtained, based on the intensity of a interference light beam obtained by combining the reflected light beam and the reference light beam (refer to U.S. Pat. Nos. 6,564,089, 6,615,072, 6,687,010 and 7,133,138, for example).
There are some optical tomographs that utilize TD-OCT (Time Domain OCT) measurement. In TD-OCT measurement, the measuring position in the depth direction (hereinafter, referred to as “depth position”) within a measurement target is changed, by changing the optical path length of the reference light beam. Thereby, tomographic images can be obtained at different depth positions within measurement targets.
As another type of optical tomograph that can obtain tomographic images at high speeds without changing the optical path length of the reference light beam, optical tomography apparatuses that employ SD-OCT (Spectral Domain OCT) measurement have been proposed. The SD-OCT optical tomograph divides a wide band low coherence light beam into a measuring light beam and a reference light beam by a Michelson interferometer. Then, the measuring light beam is irradiated onto a measurement target, and a reflected light beam, which is the measuring light beam reflected by the measurement target, is combined with the reference light beam, to obtain a interference light beam. Thereafter, the interference light beam is decomposed into different frequency components. The channeled spectra of the decomposed interference light beam undergo Fourier analysis, and tomographic images are obtained without scanning in the depth direction (refer to “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles”, R. Huber et al., OPTICS EXPRESS, Vol. 13, No. 9, pp. 3513-3528, 2005, for example).
Further, an optical tomograph that obtains optical tomographic images at high speeds without changing the optical path length of a reference light beam, by SS-OCT (Swept Source OCT) measurement, has also been proposed. The SS-OCT optical tomograph sweeps the frequency of a laser beam which is emitted from a light source. Reflected light beams of each wavelength are caused to interfere with the reference light beam. The intensities of reflected light beams at a depth positions within a measurement target are obtained by performing Fourier analysis on interference spectra for the series of wavelengths. The tomographic images are obtained employing the detected intensities.
When the various optical tomographs described above are applied to endoscopes, optical fibers are provided within probes which are to be inserted into body cavities, and light beams are guided through the optical fibers. In optical tomographs, it is often the case that a tomographic image is obtained along a predetermined surface of a measurement target. In order to do so, it is necessary to scan the light beam around the peripheral direction of the probe. There is a known structure in which a probe is configured to be rotatable in the axial direction thereof.
Meanwhile, there are cases in which living tissue and the like exhibit birefringence or optical rotatory power. There are known optical tomographs that measure the polarization state of reflected light beams when light is irradiated onto living tissue or the like, in order to investigate these types of polarization properties (refer to Japanese Unexamined Patent Publication No. 2002-301049, “Three dimensional polarization sensitive OCT of human skin in vivo”, M. Pircher et al., OPTICS EXPRESS, Vol. 12, Issue 14, pp. 3236-3244, 2004, and “Determination of the depth-resolved Stokes parameters of light backscattered from turbid media by use of polarization-sensitive optical coherence tomography”, J. F. de Boer et al., OPTICS LETTERS, Vol. 24, No. 5, pp. 300-302, 1999, for example).
When the various types of optical tomographs described above are applied to endoscopes, lasers are generally used as light sources, and linearly polarized light beams having predetermined polarization directions are employed as the measuring light beam and the reference light beam. It is preferable for the reflected light beam and the reference light beam to be adjusted when they are combined, because the intensity of the interference light beam becomes maximal when the polarization directions of the reflected light beam and the reference light beam are matched. However, single mode optical fibers, which are generally used in endoscopes, cannot necessarily maintain the polarization state of light that propagates therethrough. Therefore, when variation factors such as vibration during rotation of the probe and temperature changes are applied, the polarized state of the light that propagates through the optical fiber changes. For example, even if the light beam emitted from a light source is linearly polarized light, the polarization direction may change, or a portion of the light beam may become elliptically polarized light. That is, the polarized state of light that propagates through the optical fibers is unstable.
In addition, optical components which are employed in optical tomographs, such as mirrors and fiber couplers, have polarization properties such that the transmittance, reflectance, or the dividing ratio thereof changes according to the polarization direction of light incident thereon. In the case that light having an unstable polarization state enters optical components having polarization properties, the signal level received by a detector varies, the S/N ratio decreases, and values that do not accurately reflect the results of measurement are obtained. As a result, the image quality of tomographic images deteriorates, such as by the images becoming grainy, and targets of diagnosis which should be discriminated may be overlooked. This is a problem that arises not only when measuring the polarization properties of a measurement target, but also during general measurements.
Use of a polarization maintaining optical fiber, which is capable of propagating linearly polarized light while maintaining the polarization direction thereof, within the probe may be considered. In this case, it becomes necessary to cause the linearly polarized light beam to enter the polarization maintaining optical fiber such that the polarization direction thereof matches the unique polarization axis thereof. However, if the probe is rotated to perform scanning in the peripheral direction as described above, the polarization maintaining optical fiber therein also rotates. Therefore, it is not possible for the polarization direction of the linearly polarized light and the polarization axis of the polarization maintaining optical fiber to be constantly matched. If the polarization direction and the polarization axis do not match, the linearly polarized light that enters the polarization maintaining optical fiber often becomes elliptically polarized light, which is unsuited for measurement of polarization properties. In addition, signal levels will vary, because the polarization state of emitted light changes along with the rotation of the probe, and the image quality of tomographic images will deteriorate.
A linearly polarized light beam may be converted into a nonlinearly polarized light beam prior to entering the polarization maintaining optical fiber. A polarizing plate that transmits only light having a predetermined polarization direction may be provided. The polarizing plate may be caused to rotate along with the rotation of the probe, thereby matching the polarization axis of the polarization maintaining optical fiber within the probe and the polarization direction of the light beam. However, if this configuration is adopted, light having polarization directions other than the predetermined polarization direction is absorbed or reflected, and therefore, the amount of light loss becomes great.
The optical tomograph disclosed in U.S. Pat. Nos. 6,564,089 and 6,615,072 is provided with a Faraday rotator as an element for adjusting polarization directions. However, the Faraday rotator must be miniaturized in order to be provided at the tip of the probe, which is to be inserted into body cavities. The types of Faraday rotators which are capable of being miniaturized are limited in the wavelengths for which they can be utilized, and therefore are not suited for optical tomographs. A magnetic garnet monocrystal, in which the crystal itself has magnetism, may be employed as the material of the Faraday rotator to achieve some degree of miniaturization. However, this type of Faraday rotator is likely to generate ghosts due to reflection, because the refractive index of the magnetic body is high. For this reason, reflection preventing measures, such as provision of a watertight seal filled with index matching fluid, forming bonding surfaces at angles other than right angles to prevent feedback of reflected light, and the like, become necessary. The reflection preventing measures lead to increases in manufacturing costs.
The optical tomograph disclosed in U.S. Pat. Nos. 6,564,089 and 6,615,072 also employs a polarization controller as an element for adjusting polarization directions. The optical tomograph disclosed in U.S. Pat. Nos. 6,687,010 and 7,133,138 uses a polarization maintaining optical fiber capable of maintaining the polarization state of light for a portion of the optical path, uses single mode optical fibers for the rest of the optical path, and employs a polarization controller to adjust the polarization directions of light. However, polarization controllers are mechanically driven, which results in slow operating speeds. Other shortcomings of tomographs that employ polarization controllers are that: the tomographs become bigger in size; they are unstable because of their high sensitivity; it takes time to find optimal combinations of operational parameters, as there are three parameters to be adjusted; adjustments by operators are required, because the propagating state of light within optical fibers change; and the like. That is, these tomographs are not well suited for practical use. Particularly regarding adjustment by polarization controllers, there is the aforementioned problem of control speed thereof. Therefore, if the polarization direction shifts greatly during diagnosis utilizing OCT measurement, there is a possibility that diagnosis will be interrupted.